The present invention relates to supersonic compressors of the type comprising a channel diffuser and a rotor or impeller designed to supply fluid at an absolute velocity at least equal to Mach 1.2 to the diffuser at the design operating point, the diffuser comprising a plurality of vanes supported by a casing, distributed at evenly angular intervals and defining intervane channels having a throat section located downstream of the leading edges of the vanes.
The invention is particularly suitable for use in the field of centrifugal compressors which are practically the only ones presently used for delivering a supersonic flow to the stator portion of the compressor. Reference will consequently be made to such centrifugal compressors. However, the invention may also be applied to diffusers forming stator rings for axial compressors receiving a supersonic flow and having a high pressure ratio per stage, typically greater than 2.
Supersonic centrifugal compressors may be designed to deliver a high flow rate per unit front area and may achieve high compression rates, possibly exceeding 10. However, such result is conditioned by high circumferential speeds, typically of about 600 m/s at the blade tips for air compression rates of about 10.
Such high specific flow rate (ratio of volume flow rate to the frontal sectional area of the disc of the rotor) and compression rate (for example 10) can be attained only with a relative speed at the tips of the blades at the rotor input which is highly supersonic. In the case contemplated and in the case of a rotor having blades which exhibit a ratio between the output radius and the input tip radius equal to 1.5, the relative input Mach number at the blade tip will be of about 1.3.
The existence at the input of the diffuser of zones where the absolute speed is supersonic strongly limits the range of variations of the volume flow rate which may be accepted by the diffuser and consequently the volume flow rate range at the input to the compressor. The existence of zones where the relative speed is supersonic at the input of the rotor also limits the volume flow rate range, but the latter limitation is less stringent than the first one when the flow is primed at the throat of the intervane channels in the diffuser.
The impact of the first limitation is such that, as soon as Mach numbers exceeding about 1.25 are reached at the input of the diffuser, the extent of the volume flow rate range is drastically reduced and the compressor can only operate at one predetermined flow rate.
It is an object of the invention to provide an improved supersonic compressor, particularly a centrifugal compressor in which the diffuser receives fluid at a speed typically exceeding a Mach number of 1.2 under rated or design conditions, which has an extended flow rate range. It is another object of the invention to provide a centrifugal compression whose rotor has characteristics of self matching to the diffuser when the compressor operates under variable flow rate conditions.
For that purpose, the invention includes a compressor of the above-defined type in which each intervane channel of the diffuser has two parietal slots whose length in the flow direction is such that they extend on each side of the throat section of the channel, all slots situated on the same side of the channel communicating with a common volume through passages whose cross-sectional area is at least equal to that of the slots throughout the length of the passages.
The invention will be better understood from the following description of particular embodiments thereof, given by way of examples only, and from the comprison which is made with the prior art.